Spleen

The spleen (splen) is the largest of the lymphatic organs. It is located in the upper left portion of the abdominal cavity, just inferior to the diaphragm, posterior and lateral to the stomach (see fig. 16.12 and reference plates 4, 5, and 6).

The spleen resembles a large lymph node in that it is enclosed in connective tissue extending inward from the surface and partially subdividing the organ into chambers, or lobules. The organ also has a hilum on one surface through which blood vessels and nerves enter. However, unlike the sinuses of a lymph node, the spaces (venous sinuses) within the chambers of the spleen are filled with blood instead of lymph.

The tissues within the lobules of the spleen are of two types. The white pulp is distributed throughout the spleen in tiny islands. This tissue is composed of nodules (splenic nodules), which are similar to those in lymph nodes and contain many lymphocytes. The red pulp, which fills the remaining spaces of the lobules, surrounds the venous sinuses. This pulp contains numerous red blood cells, which impart its color, plus many lymphocytes and macrophages (figs. 16.14 and 16.15).

Blood capillaries within the red pulp are quite permeable. Red blood cells can squeeze through the pores in these capillary walls and enter the venous sinuses. The older, more fragile red blood cells may rupture as they make this passage, and the resulting cellular debris is removed by macrophages within the splenic sinuses.

Splenic artery Splenic vein

Splenic artery Splenic vein

Blood Vessels Spleen Lymphatic Tissue

Capillary

Figure 16.14

The spleen resembles a large lymph node.

Capsule

Artery of pulp

White pulp Venous sinus

Red pulp

Figure 16.14

The spleen resembles a large lymph node.

Capsule

Capillary

Connective tissue

Red pulp

White pulp

Red pulp

White pulp

Passage Rbcs Spleen

Capsule

Figure 16.15

Light micrograph of the spleen (15x).

Capsule

Figure 16.15

Light micrograph of the spleen (15x).

During fetal development, pulp cells of the spleen produce blood cells, much as red bone marrow cells do after birth. As the time of birth approaches, this splenic function ceases. However, in certain diseases, such as erythroblastosis fetalis, in which many red blood cells are destroyed, the splenic pulp cells may resume their hematopoietic activity.

The macrophages also engulf and destroy foreign particles, such as bacteria, that may be carried in the blood as it flows through the sinuses. The lymphocytes of the spleen, like those of the thymus, lymph nodes, and nodules, also help defend the body against infections. Thus, the spleen filters blood much as the lymph nodes filter lymph. Table 16.1 summarizes the characteristics of the major organs of the lymphatic system.

H Why are the thymus and spleen considered organs of the lymphatic system?

What are the major functions of the thymus and the spleen?

Organ

Location

Function

Lymph

In groups or chains

Filter foreign particles

nodes

along the paths

and debris from

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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